The present invention relates generally to magnetic resonance imaging (MRI), and more particularly, to a switchable field-of-view (FOV) apparatus and method to reduce the occurrences of wrap around artifacts during image reconstruction.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field along a Z-axis, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in an x-y plane and which is near the Larmor frequency, the net aligned moment, or xe2x80x9clongitudinal magnetizationxe2x80x9d, MZ, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (GxGy and Gz) are employed using gradient coils. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MRI signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
The transmission of a radio frequency (RF) excitation pulse through a subject and the reception of the resulting resonant signal is known in the art of nuclear magnetic resonance imaging. Examples of structures capable of transmitting and receiving RF pulses include a helical coil, saddle coil, resonant cavity, and a birdcage resonator or coil. While the use of these structures for transmission and reception of image signals has greatly improved reconstruction of an image, there are some existing drawbacks to the current designs. For example, the occurrence of wrap around artifacts can create distortion during the image reconstruction process. Due to the non-linearity of the gradient coils and the in-homogeneity of the uniform background magnetic field B0, there are areas outside the FOV that resonate at the same frequency as portions inside the FOV, such that aliasing wrap around artifacts are displayed upon image reconstruction. These undesirable wrap around artifacts cause a portion of the imaged subject, which is located outside the FOV, to appear inside of the FOV as part of the volume imaged.
The birdcage resonator and other structures cause limitations as to the strengths of magnetic fields introduced in the system because of their whole-body imaging methodology. Whole-body coils cause more irradiation of patient volume than equivalent shorter length coils. Irradiation levels are regulated according to an average specific power absorption rate (SAR) per unit mass for patients under examination. As a result, there is a need for an apparatus capable of operating with increased magnetic field strength without exceeding regulated absorption rates that also reduces the occurrence of wrap around artifacts during image reconstruction.
It would therefore be desirable to have an RF coil and control apparatus to restrict sensitivity in areas outside a selected FOV, thereby reducing the occurrence of wrap-around artifacts caused by extended areas of a whole-body coil while maintaining good sensitivity and homogeneity within the FOV.
The present invention provides a switchable FOV magnetic resonance imaging apparatus and method solving the aforementioned drawbacks.
The invention includes the use of a magnet to produce a magnetic field for MRI imaging of a subject. After the subject is placed within a bore of the magnet having a uniform linear magnetic field so that nuclei within the subject are excited and encoded using a set of magnetic field gradient coils, a desired FOV size is selected by an operator. This selection causes an automatic determination and activation of a center coil only or the center coil and a pair of end coils in combination for transmission of a signal desired to perform imaging of a subject. During activation of the center coil only, the effective longitudinal length of the RF coil is less causing a reduction in wrap around artifacts as compared to a standard whole-body coil. A shorter RF coil causes limited excitation of nuclei in areas outside the desired FOV.
In accordance with one aspect of the invention, an RF coil and control configuration includes an RF coil assembly having a center coil and a pair of end coils. Each end coil is comprised of at least one RF coil, but can include more. The RF coil assembly is configured to encode and excite spins over an adjustable FOV. With activation of both the center coil and the pair of end coils, an imaging scan can be acquired that is comparable to a whole-body coil imaging scan. A control is connected to the RF coil assembly to switch the RF coil assembly between at least two general FOV sizes based on an FOV size input from an operator. The at least two general FOV size designations are determined by the control switching power to activate either a center coil alone or the center coil and the pair of end coils in unison.
In accordance with another aspect of the invention an MRI apparatus to acquire images is disclosed having an MRI system having a plurality of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field B0 through a subject under examination. An RF transceiver system and an RF switch controlled by a pulse module are included to transmit RF signals to an RF resonator assembly having a center resonator enclosed by a set of end resonators to acquire magnetic resonance (MR) images of the subject. A computer is further included and programmed to receive an FOV input and based on that input, cause transmission of a signal from either the center resonator only or the center resonator and the end resonators simultaneously.
In accordance with yet another aspect of the present invention, a technique of controlling an MRI device having a central RF coil assembly comprised of a central RF coil and at least one pair of end RF coils includes the steps of creating a predetermined parameter based on a length of the central RF coil and then comparing a desired FOV to the predetermined parameter. If the desired FOV is not greater than the predetermined parameter, then the central RF coil is selected for energization so as to perform an image scan of the desired FOV. If the FOV is greater than the selected parameter, then the central RF coil and the at least one pair of end RF coils are selected for energization thereby performing an imaging scan of the desired FOV.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.